The invention relates to a hydrostatic propulsion drive.
In the article "Hydraulik im Forst" by Dipl.-Ing. Klaus Hogemann, which appeared in special publication number 1/84 of the firm Mannesmann Rexroth, there is described a hydrostatic propulsion drive which drives a mobile forestry vehicle and has so-called limit load control and a so-called pilot control.
With the described limit load control, the rate of travel varies with the speed (speed of rotation) of the drive motor, determined by means of the gas pedal, and with the displacement volume of the hydraulic pump which is automatically adjusted in dependence upon a setting pressure proportional to this speed of the drive motor. The hydrostatic propulsion drive is in this case located in the so-called automotive or automatic traction means, which is mainly employed for goods and on-road transportation.
In the case of pilot control the rate of travel is determined, at constant speed of the drive motor, solely by means of the displacement volume of the hydraulic pump which is adjusted in dependence upon the volume of the pressure medium, subjected to the speed-dependent setting pressure, flowing into the setting device. This pilot control permits sensitive control of the rate of travel by means of a hand controller, which sensitive control is particularly important in the operational employment of the vehicle.
The volume control of the pressure medium during the pilot control is achieved with the aid of the mechanical follow-up device which includes a follow-up lever which is attached to the setting piston of the setting device constituted as a setting cylinder and to the control sleeve of the pilot valve and with the pilot valve opened in correspondence to the position of the hand controller transmits the setting movement of the setting piston to the control sleeve for so long that the pilot valve again attains its closed position.
Through further opening of the pilot valve by means of the hand controller, beyond the maximum travel of the follow-up lever, the follow-up is made ineffective and the pilot valve brought into the through-flow end position in which the pressure medium, subjected to the speed-proportional setting pressure, flows for so long into the setting cylinder until equilibrium of forces is effective at its spring loaded setting piston. The hydrostatic propulsion drive is in this way switched from pilot control to limit load control.
Because of the dependence of the displacement volume of the hydraulic pump upon the speed of the drive motor, corresponding to the relevant characteristic line, when travelling with the pilot control, i.e. when travelling with constant drive motor speed, the setting pressure corresponding to this constant speed limits the setting movement of the setting piston and thus the maximum displacement volume of the hydraulic pump, which determines the rate of travel and the torque. Thus, the travel region during the pilot control corresponds to the region below the characteristic line. This is disadvantageous to the extent that high propulsing torques are only available with correspondingly high speed of the driving motor and thus high rate of travel.